It is standard practice to construct cement walls by first erecting a system of forms defining a space into which concrete is poured. The forms are essentially panels reinforced by various patterns of beams, the dimensional relationships being determined primarily by the depth of the pour and the resulting pressure of the poured concrete mass. Where the forms are intended for extensive re-use, panels of plywood or sheet steel are frequently provided with marginal steel beams to provide a modular unit that can be interconnected to form a system of any desired length.
Poured concrete will generate pressure of approximately 160 pounds per square foot per foot of depth, with the inevitable result that forms of any substantial size will require securing systems of tremendous strength. The most reliable of these involves an imbedded tie rod extending across the space between the forms, and which is submerged when the concrete is poured. This tie rod is threaded at both ends, and is engaged by bolts traversing the form panels and secured to the reinforcing beams supporting the panels. The pressure against each of the opposite form systems is thus equalized. The cross-sectional dimensions of each of the tie assemblies is inter-related with the spacing of them so that the concrete pressure is effectively resisted.
Form systems are used primarily by commercial contractors, and the overall cost of the procurement and use of the forms is therefore highly important. The initial cost of the form structure, having in mind the degree of re-use for which it is designed, must be considered along with the erection time of the form determined primarily by its tie system, and also the stripping time required to disengage the tie system and remove the forms from the completed concrete wall. Accuracy of spacing is obviously important, in order to preserve the dimensional continuity of the wall surface.